Courtesy of Steelfistv:

/script local a,b=SetCVar,"UnitNameOwn"if GetCVar(b)=="1" then a(b,0)else a(b,1)end

Change UnitNameOwn to any of these to toggle;

UnitNameOwn - (0/1) Toggle Own Name
UnitNameNPC - (0/1) Toggle NPC Name
UnitNamePlayerGuild - (0/1) Toggle Guild Tags
UnitNamePlayerPVPTitle - (0/1) Toggle Titles
UnitNameFriendlyPlayerName - (0/1) Toggle Friendly Player Names
UnitNameFriendlyPetName - (0/1) Toggle Friendly Pet Names
UnitNameFriendlyCreationName - (0/1) Toggle Friendly Creation Names
UnitNameEnemyPlayerName - (0/1) Toggle Enemy Player Names
UnitNameEnemyPetName - (0/1) Toggle Enemy Pet Names
UnitNameEnemyCreationName - (0/1) Toggle Enemy Creation Names
UnitNameCompanionName - (0/1) Toggle Companion Names